1. Field of the Invention
The present invention relates to a method of impact plating selective metal powders onto metallic articles.
2. Description of the Related Art
Impact plating, otherwise known as mechanical plating, is a well-established technique for applying powdered coating materials onto discrete articles. For example, U.S. Pat. Nos. 2,640,001 and 2,640,002 disclose methods of applying finely divided metal powder onto discrete articles, such as screws and nails, by placing the articles, the metallic powder, film-forming organic compounds and, optionally, metallic balls, in a rotating mill. The rotation of the mill causes the metal powder to be impacted into a coating on the articles.
The use of water and water soluble organic compounds, rather than a completely organic medium, to plate finely divided metallic powder onto discrete articles is disclosed in U.S. Pat. No. 2,689,808. This type of aqueous environment containing various water soluble organic compounds is also disclosed in other patents such as U.S. Pat. Nos. 3,023,127 and 3,132,043.
As the art of impact plating has progressed, several developments concerning different aspects of the process have been made. For instance, U.S. Pat. Nos. 3,141,780 and 3,164,448 describe the pretreatment of the metallic articles with a flash coating of copper in order to improve the adhesion between the article and the subsequently impact plated metallic powder.
Developments in the chemicals used to obtain the chemical plating, which are commonly known as "promoter chemicals" or "plating accelerators", are marked by such patents as U.S. Pat. No. 2,999,767 wherein various organic ammonium chloride salts are employed to facilitate the plating of brass powder onto lead articles, U.S. Pat. No. 3,328,197 wherein high molecular weight polyoxyethylene glycol is used as a promoter chemical, U.S. Pat. No. 3,460,977 wherein a variety of defined dispersants are used as a promoter chemical and U.S. Pat. No. 3,479,209 wherein water insoluble oxygen-substituted lubricious aromatic compounds are employed as the promoter chemical.
The impact media which is commonly used in the impact plating process has also undergone a transition. In particular, whereas in the earlier techniques, metallic balls were commonly employed, U.S. Pat. No. 3,251,711 describes non-metallic impacting granules which are vitreous, ceramic or mineral in nature, while U.S. Pat. Nos. 3,013,892 and 3,443,985 describe the use of cullet or glass beads, respectively, as the impact media.
With respect to process developments, U.S. Pat. No. 3,268,356 sets forth a technique wherein metallic particles and/or chemical plating promoter is added over substantial portion of the plating cycle. In U.S. Pat. No. 3,400,012, flash coatings are provided on conductive substrates by employing a dissolved metal salt and a driving or plating inducing metal. In U.S. Pat. No. 3,531,315, the articles are plated by successively adding the necessary chemicals into a rotating barrel in the absence of any intervening rinsing operation. More recently, U.S. Pat. No. 3,690,935 discloses a process wherein all the water is recovered and reused and U.S. Pat. No. 4,062,990 sets forth a process wherein all treating and plating chemicals are recovered and reused.
From an apparatus development standpoint, U.S. Pat. Nos. 3,442,691 and 3,494,327 disclose plating systems wherein the plating barrel rotates and vibrates at the same time. U.S. Pat. Nos. 3,726,186 and 4,162,680 disclose the apparatus aspects of the previously mentioned processes wherein water or all the treating and plating materials are captured and reused.
Despite the development of numerous aspects of impact plating, the technique has been practically limited to plating the metals tin, cadmium, zinc or combinations of tin, cadmium and/or zinc onto various substrates. Notwithstanding descriptions in the art that any malleable metal can be impact plated, there has been no known commercial way of impact plating metals such as aluminum, brass or stainless steel. The present invention is directed to a solution of this problem.